The present invention relates to a method of producing hydrated iron oxide which is well suited for use as a starting material for producing an acicular or needle-like magnetic iron oxide powder or acicular ferromagnetic metal powder used for magnetic recording media.
Generally, an acicular magnetic iron oxide powder or acicular ferromagnetic metal powder for magnetic recording media is produced by heating and reducing a hydrated oxide of iron (Fe.sub.2 O.sub.3.H.sub.2 O) by hydrogen.
Since the particle shape of such magnetic iron oxide powder or ferromagnetic metal powder is much dependent on the shape of a starting material, i.e., a hydrated oxide of iron (.alpha.-FeOOH, .beta.-FeOOH or .gamma.-FeOOH), the shape of a hydrated iron oxide used as a raw material is very important.
In the past, a hydrated iron oxide production method has been used widely in which a hydrated iron oxide is produced through the reaction of a ferrous salt solution with an alkali and an oxygen-containing gas.
While this method is advantageous in that the acicular form (the length-to-width ratio) of the produced hydrated iron oxide is excellent and that the particle size of the hydrated iron oxide can be controlled relatively easily, it is also disadvantageous in that branching tends to occur easily and that the range of a particle-size distribution is wide. On the other hand, a method has been proposed in which a hydrated iron oxide is produced by ageing a suspension of ferric hydroxide obtained by neutralizing a ferric salt with an aqueous solution of alkali. While the hydrated iron oxide of this method has the advantage of eliminating the branching, there are disadvantages that it is difficult to control the particle size and particularly it is difficult to obtain the fine particle and that the range of a particle-size distribution is wide.